1. Field of the Invention
The present invention relates to an optical fiber coating method and apparatus therefor, which can form a high quality coating layer on an optical fiber which is drawn at high speed.
2. Description of the Related Art
An optical fiber drawn from a drawing furnace is normally provided a primary coating to be a buffer layer with an ultraviolet curing resin, silicon resin or the like, before contacting with a solid substance, such as a take-up roller. Furthermore, a secondary coating layer for reinforcement is provided with an ultraviolet curing resin, nylon or the like.
As such conventional optical fiber coating apparatus, the apparatus disclosed in Japanese Patent Application Publication No. 2-25854 (1990), Japanese Patent Application Laid-Open No. 61-72656 (1986), Japanese Patent Application Publication No. 5-37938 (1993), Japanese Patent Application Laid Open No. 64-65430 (1989), for example, have been known.
The apparatus disclosed in Japanese Patent Application Publication No. 2-25854 detects a center position of an optical fiber drawn from a drawing furnace by means of a measuring unit. Then, the overall coating apparatus is shifted along a plane perpendicular to an axis of the optical fiber by means of a motor so that it may be aligned with the optical fiber coaxially. Thereby, the coating layer of uniform thickness is formed on the optical fiber.
The apparatus disclosed in Japanese Patent Application Laid-Open No. 61-72656 measures an external diameter of the optical fiber with a coating resin layer after passing through a coating die set by an external diameter measuring device. On the basis of the measured value, a supplying pressure of a resin supplied in the coating die set is controlled. Thereby, the optical fiber having a coating layer of uniform external diameter can be obtained.
The apparatus disclosed in Japanese Patent Application Publication No. 5-37938 detects concentricity of a coating resin layer applied by a coating application device with respect to the optical fiber by means of a monitoring device. On the basis of result of detection, the coating application device is inclined about an orthogonal two axes (these two axes are located on a plane perpendicular to the axis of the optical fiber) taking a lower side outlet orifice of the coating application device as a center. Thereby, non-concentricity of the coating resin layer with respect to the optical fiber can be restricted.
In the apparatus disclosed in Japanese Patent Application Laid-Open No. 64-65430, the optical fiber on which the primary coating resin in an upper tank have been supplied, is guided in a pool of a secondary coating resin which is supplied into the lower tank. Then, a transmitted illumination light is projected to the optical fiber to detect the non-concentricity of the primary coating resin layer. On the basis of this, the position of the upper tank relative to the optical fiber is corrected so as to reduce magnitude of non-concentricity.
If the thickness of the primary coating layer or the secondary coating layer formed on the optical fiber is non-uniform, a microbending phenomenon due to variation of ambient temperature, external force applied on the optical fiber or the like, namely, irregular bending, may be caused on the optical fiber resulting in increase transmission loss of the optical fiber. Also, when the thickness of the primary coating layer or the secondary coating layer is non-uniform, strength at the portion where the coating layer is thin, becomes relatively low to increase tendency to snap at such portion. Furthermore, if a bubble is admixed in the coating resin layer, the strength of the optical fiber at such portion becomes relatively low. In addition, since the volume of the bubble varies depending upon variation of the ambient temperature, the foregoing microbending phenomenon is caused to increasing transmission loss of the optical fiber.
Recently, as optical fiber preform have expanded in length, drawing speed is becoming higher. When the optical fiber is drawn at such high speed, it may be possible to reduce non-concentricity of the coating resin layer by the conventional coating apparatus disclosed in Japanese Patent Application Laid-Open No. 64-65430. However, it is inherently required to detect the magnitude of non-concentricity as a numerical value which is difficult to calculate. Furthermore, it is not possible to make the thickness of the coating resin layer uniform or to prevent bubbles from being included in order to form the high quality coating layer. In addition, since the optical fiber after applying and curing the primary coating resin is observed in the secondary coating resin, a dedicated observing means becomes necessary for measuring the magnitude of non-concentricity to observe the optical fiber after secondary coating. As a result, facility becomes large.
On the other hand, in the invention disclosed in Japanese Patent Application Publication 2-25854, an adjustment for the non-concentricity consisting of shifting the magnitude of the coating device is not sufficient to is adjust uniformity sufficiently. Since fluctuation of the thickness of the coating resin layer does not depend on variation of the supplying pressure, it is not possible to maintain a uniform layer thickness by the method disclosed in Japanese Patent Application Laid-Open No. 61-72656. In the invention disclosed in Japanese Patent Application Publication No. 5-37938, the concentricity monitoring device is required but there is no disclosure for how the fluctuation of the external diameter of the coating resin layer can be restricted.
Furthermore, in these conventional apparatuses, associated with increasing of drawing speed of the optical fiber, delicate variation of the drawing condition, such as temperature or drawing tension of the optical fiber, and supplying pressure or temperature of the coating resin, which has not been significantly affected upon drawing at low speed, should affect for flow condition of the resin to be applied. As a result, non-concentricity of the coating resin layer or non-uniformity of the layer thickness can be caused, and admixing of bubble in the coating resin layer is potentially caused.
Particularly, when the drawing speed of the optical fiber becomes higher, the thickness of the coating resin layer can fluctuate to create an unstable condition. Simultaneously, the non-concentric condition becomes unstable as often as not, and the applying condition of coating resin layer become unstable. In such case, there is not suggested or disclosed as to how the coating device is to be adjusted in the conventional apparatus.
An object of the present invention is to provide an optical fiber coating method which can obtain a high quality coating layer on an optical fiber by reducing non-concentricity or fluctuation of thickness of a coating resin layer applied to the optical fiber drawn at high speed and by preventing admixing of bubbles.
Another object of the present invention is to provide an optical fiber coating apparatus which can realize such optical fiber coating method.